The present invention relates to fuel nozzles for gas turbines. More specifically, the present invention relates to a fuel nozzle assembly having a replaceable nozzle cap.
Gas turbines include a combustion system having one or more combustors adapted to produce a hot gas by burning a fuel in compressed air. A fuel nozzle assembly is employed to introduce the fuel into each combustor.
Traditionally, a fuel nozzle is comprised of a base portion and a nozzle cap. The base portion has an inlet port that receives the fuel to be burned and that secures the fuel nozzle assembly to the combustion system, either by bolting to the combustor itself or to a cylinder enclosing the combustors. The nozzle cap features fuel outlet ports that serve to inject the fuel into the combustor. Typically, the nozzle cap extends from the nozzle base so as to enter into the combustor. Because of the proximity of the nozzle cap to the flame front and the hot combustion gases within the combustor, the nozzle caps are subject to deterioration due to burning, erosion and corrosion. Consequently, the nozzle caps must be replaced relatively frequently.
Since gas turbines can operate on a variety of fuels, including both liquid and gaseous fuels, and may require the injection of steam into the combustor to minimize the formation of NOx, an environmental pollutant, modern fuel nozzle assemblies must be capable of introducing two fluids into the combustor. Thus, to facilitate rapid switching from one fuel to another, fuel nozzle assemblies are often manufactured in a "dual fuel" configuration--thereby avoiding the necessity of changing nozzles when changing fuels. In addition, fuel nozzle assemblies are also manufactured in a gas/steam configuration for emissions control.
As a result of the requirement that the fuel nozzle assembly be capable of introducing two different fluids in the combustor, modern fuel nozzle assemblies have relatively complex internal passages. Typically, inner and outer sleeves connect the nozzle cap to the base and form an annular passage therebetween that directs the gas fuel from the inlet port formed in the nozzle base to the outlet port formed in the nozzle cap. In addition, the inner sleeve forms a central cavity therewithin that houses an oil spray nozzle, in the case of dual fuel gas/oil nozzle assembly, or that forms a passage that directs steam from an inlet port formed in the nozzle base to an outlet port formed in the nozzle cap, in the case of a gas/steam nozzle assembly. As a result of this complex geometry, in the traditional arrangement, the nozzle base and cap were manufactured as parts of a unitary cast or welded structure.
As a result of this unitary structure, replacement of the nozzle cap requires machining the old nozzle cap from the base and welding on a new cap. This work demands specialized tooling and trained personnel. Consequently, it is necessary to transport the nozzles to an off-site repair facility. The need to remove the nozzles from the power plant considerably increases the cost and downtime associated with maintenance of the combustion system and represents a maintenance problem for the user.
It is therefore desirable to provide a fuel nozzle assembly for a gas turbine that is capable of burning more than one fuel, or burning gaseous fuel along with injecting steam, and that allows the nozzle cap to be readily separated from the nozzle body so that nozzle cap replacement can be readily performed by the user.